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General labeling of ripe mouse spermatozoa was obtained two weeks after six injections of L-arginine-5-3H monohydrochloride, administered within two days. Besides tail labeling a very intensive nuclear labeling was obtained. It is presumed that this nuclear labeling represents almost exclusively tritiated arginine incorporation into basic nuclear protein. Mouse oocytes were fertilized in vitro with such labeled spermatozoa. It was found that the nuclear label is lost very early during male pronucleus formation, probably concomitantly with sperm chromatin decondensation. This might indicate total loss of the basic spermatozoal nuclear protein before male genome activation.

The development of fertilized ova in explanted parts of oviducts was studied from 12 to 36 hours after fertilization, and from 36 to 84 hours after fertilization. No egg mortality during cultivation, nor any slowing of development could be detected in either experiment. It may be presumed that the mortality of mouse ova during the period before implantation has no essential effect on the resulting fertility. During cultivation of oviducts for 144 hours, there occurred in most blastocysts herniation of their walls resulting in a formation similar to the blastocyst, and a migration of a disorganized mass of cells, some of which resembled the giant trophoblastic cells. After transplantation of ova fertilized in vitro to recipients, in four cases there occurred nidation and subsequent normal development of the embryo.